The most frequently encountered problem with voltage-signalling UPSes (e.g. BackUPS 650) is that you have incorrectly specified which cable is being used. All cables furnished by APC have the cable number stamped on the side of the computer connector end of the cable. Using this number with apcupsd will normally work fine. If you do not know what cable you have, you can use the apctest program to determine the type of the cable.
For simple signaling UPSes, you should not use simple in the cable specification (i.e. UPSCABLE simple) unless you have made the cable yourself according to the wiring diagram given in the cables chapter of this manual.
Once you have established that apcupsd can talk to the UPS over the serial part, go do the series of functional tests described in the main Testing (see Testing Apcupsd) section.
One additional note applies:
In one case, a user reported that he received random incorrect values from the UPS in the status output. It turned out that gpm, the mouse control program for command windows, was using the serial port without using the standard Unix locking mechanism. As a consequence, both apcupsd and gpm were reading the serial port. Please ensure that if you are running gpm that it is not configured with a serial port mouse on the same serial port.